Surface State Density Determines the Energy Level Alignment at

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Surface State Density Determines the Energy Level Alignment at Hybrid Perovskite / Electron Acceptors Interfaces Feng-Shuo Zu, Patrick Amsalem, Maryline Ralaiarisoa, Thorsten Schultz, Raphael Schlesinger, and Norbert Koch ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12586 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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ACS Applied Materials & Interfaces

Surface State Density Determines the Energy Level Alignment at Hybrid Perovskite/Electron Acceptors Interfaces

Fengshuo Zu,a,b Patrick Amsalem,*a Maryline Ralaiarisoa,a Thorsten Schultz,a Raphael Schlesingera and Norbert Koch*a,b a

Institut für Physik & IRIS Adlershof, Humboldt-Universität zu Berlin, 12489 Berlin,

Germany b

Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, 12489 Berlin,

Germany

KEYWORDS halide perovskite (CH3NH3PbI3-xClx) films, CH3NH3PbI3 single crystals, X-ray and ultraviolet photoelectron spectroscopy, density of surface states, energy level alignment

ACS Paragon Plus Environment

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ABSTRACT

Substantial variations in the electronic structure and thus possibly conflicting energetics at interfaces between hybrid perovskites and charge transport layers in solar cells have been reported by the research community. In an attempt to unravel the origin of these variations and to enable reliable device design, we demonstrate that donor-like surface states stemming from reduced lead (Pb0) directly impact the energy level alignment at perovskite (CH3NH3PbI3-xClx) and molecular electron acceptor layer interfaces, using photoelectron spectroscopy. When forming the interfaces, it is found that electron transfer from surface states to acceptor molecules occurs, leading to a strong decrease in the density of ionized surface states. As a consequence, for perovskite samples with low surface state density, the initial band bending at the pristine perovskite surface can be flattened upon interface formation. In contrast, for perovskites with a high surface state density, the Fermi-level is strongly pinned at the conduction band edge and only minor changes in surface band bending are observed upon acceptor deposition. Consequently, depending on the initial perovskite surface state density, very different interface energy level alignment situations (variations over 0.5 eV) are demonstrated and rationalized. Our findings help explaining the rather dissimilar reported energy levels at interfaces with perovskites and refining our understanding of the operating principles in devices comprising this material.

1. INTRODUCTION Organic-inorganic hybrid perovskites have recently emerged as highly efficient


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optoelectronic materials and have attracted enormous research interest, particularly in photovoltaics (PVs).1-3 Within only a few years of research, perovskite solar cells prepared from low-temperature solution process have achieved more than 20% certified power conversion efficiency4 owing to their high optical absorption coefficient2 and favorable charge transport properties,5 making them one of the most promising candidates for the next photovoltaic technology. Within this class of material, the most commonly used perovskite absorbers are methylammonium lead halides (MAPbX3, X = Cl, Br, or I), which can be processed from a variety of preparation routes, spanning from thermal evaporation6 to various solution processes.7-9 A typical solar cell based on perovskite polycrystalline thin films employs two selective electron and hole transport layers, which consist of either organic7-9 or inorganic materials,10,11 or a combination thereof.12,13 Although the bulk properties of the perovskite absorber film are crucial for device performance, the interface between the absorber and the charge transport layers is also of paramount importance for efficient charge transport, which impacts the solar cell efficiency. For instance, the energy level alignment (ELA) at the perovskites/charge transport layer interface plays a key role for charge extraction14,15 and the open circuit voltage (Voc).16 In contrast to the rapid progress made in device efficiency, the understanding of the fundamental physical aspects when an interface is formed, such as the dependence of the level alignment on the actual perovskite surface properties, has not been established yet. Recently, several studies reported on the interface ELA between perovskites and


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different charge transport layers using photoelectron spectroscopy,17-22 since this technique is most widely used to investigate the core level and valence electronic structure of semiconductor surfaces and interfaces. Noteworthy, several studies have reported different and even conflicting interface ELA scenarios even for the same material combinations. For instance, Schulz et al.19 found that almost flat band conditions hold when forming the methylammonium lead iodide (MAPbI3)/C60 interface and no band bending was observed in the MAPbI3 layer; in another study on the same system, Wang et al.18 reported an upward band bending in the perovskite layer accompanied by an upward shift of the C60 energy levels. These discrepancies might be, at least partially, ascribed to different perovskite preparation conditions,23,24 which are very likely to change film properties25,26 and thus the measured interface level alignment, but clear-cut correlations have not been established. It is thus essential to rationalize the underlying mechanism for the community to reach consensus for reliable future device development. The electronic properties of mixed halide perovskites have been extensively studied and many reports show that these materials can vary drastically, from n- to p-type as a function of the preparation condition.23,27 It is also still debated whether the position of the Fermi-level as determined by photoemission is indicative of the Fermi-level both in the bulk and at the surface or at the surface only.28 In a previous study, we demonstrated that the often observed n-type character of mixed halide and single crystal perovskites29 as measured in photoemission arises due to surface downward band bending caused by donor-type surface states. Those most


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likely are reduced (also referred to as metallic) lead (Pb0), which pin the Fermi-level (EF) close to the conduction band minimum (CBM). Depending on the density of surface states (DoSS), the surface band bending can be strongly reduced upon white light illumination, i.e., the surface photovoltage.30 However, prolonged white light illumination in ultra-high vacuum (UHV) was also shown to result in an enhancement of the DoSS, leading to the quenching of the surface photovoltage due to stronger EF pinning at the CBM on the film surface. Taking these findings into consideration, we performed a detailed study of the electronic properties and the ELA at perovskite/molecular electron acceptor interfaces, using perovskite samples displaying strongly varying DoSS. These samples are i) pristine thin films with low-DoSS, ii) thin films illuminated with white light and thus very high-DoSS, and iii) single crystals also displaying relatively high-DoSS. As electron

acceptor

molecules

1,4,5,8,9,12-hexaazatriphenylenehexacarbonitrile 2,2’-(perfluoronaphthalene-2,6-diylidene)dimalononitrile

we (HATCN) (F6TCNNQ),

employ and whose

electron affinities (EAs) are higher than the typical perovskite work function (Φ). These molecules were evaporated onto the different perovskite samples and the interface electronic properties were investigated by ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS, respectively). Through these systematic studies, we demonstrate that the ELA at perovskite/molecular acceptor interfaces can be drastically modified by the density of (Pb0-related) donor-type surface states. Notably, upon adsorption of acceptor


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molecules, the density of ionized surface states is found to be largely reduced as a result of electron transfer from surface states to the molecules. Consequently, fundamentally different surface band bending scenarios are observed depending on the initial DoSS. Particularly, when the perovskite initially exhibits low-DoSS, strong band flattening resulting from an upward band bending by ca. 0.7 eV is found after molecular deposition. For samples with higher DoSS, EF is strongly pinned close to the CBM and therefore only small changes in the surface band bending are observed. Consistently, we reveal that the energy level offset between the perovskite valence band maximum (VBM) and the acceptor highest occupied molecular orbital (HOMO) level depends strongly on the initial perovskite DoSS. These results demonstrate the major role played by the perovskite surface DoSS in determining the ELA at perovskite/organic charge transport layer interfaces. Furthermore, our findings help to rationalize the discrepancies regarding the ELA reported in literature for similar interfaces. They also represent a key to ultimately optimize device architectures from the level alignment point-of-view in a reliable manner.

2. EXPERIMENTAL DETAILS 2.1. Sample Preparation The preparation of the pristine perovskite CH3NH3PbI3-xClx films and large-size single crystals (CH3NH3PbI3) can be found in the previous work.29 The crystal was cleaved with a knife in a N2-filled glove box before photoemission measurements, and all samples were transferred to the UHV system without air exposure.


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The illuminated perovskite samples were prepared by illuminating pristine perovskite films under white halogen lamp (@150 mW cm-2, daylight rendering spectrum) for 40 min in UHV condition in order to obtain high density of Pb0. Surface morphology of thin films and single crystals before and after adsorption of F6TCNNQ and HATCN molecules are provided in Supporting Information. 2.2. Acceptor Molecules Depositions HATCN and F6TCNNQ molecules were purchased from Novaled. Both materials were used as received and thermally evaporated from resistively heated quartz crucibles. The nominal deposited thickness was monitored by a quartz crystal microbalance. 2.3. Photoemission spectroscopy measurements Photoemission experiments were performed at an UHV system consisting of sample preparation and analysis chambers (both at base pressure: 1 × 10-10 mbar), as well as a load-lock (base pressure: 1 × 10-6 mbar). All the samples were transferred to the UHV chamber using a transfer rod under rough vacuum (1 × 10-3 mbar). UPS was performed using helium discharge lamp (21.22 eV) with a filter to reduce the photo flux and to block visible light from the source to hit the sample. XPS was performed using Al Kα radiation (1486.7 eV) generated from a twin anode X-ray source. All spectra were recorded at room temperature and normal emission using a hemispherical SPECSPhoibos 100 analyzer. The overall energy resolution was 140 meV and 1.2 eV for UPS and XPS,31 respectively.


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3. RESULTS AND DISCUSSION 3.1. Energy Level Alignment at Low-DoSS Perovskites/Acceptors Interfaces The electronic structure at interfaces between the molecular acceptors and low-DoSS (pristine) MAPbI3-xClx perovskite samples are presented at first. Figure 1a and b show the evolution of the secondary electron cut-off (SECO) and valence band (VB) spectra, respectively, as a function of nominal HATCN layer thickness. Initially, the pristine perovskite film has a work function of 4.34 eV and the VBM is at 1.50 eV binding energy (BE). Given the transport gap of 1.70 eV,32 this indicates the pronounced n-type character of the perovskite surface. As HATCN is incrementally deposited, Φ increases monotonically and eventually saturates at 5.49 eV for a HATCN film with 128 Å nominal thickness. This Φ increase implies a net charge transfer from the perovskite to HATCN. This established electronic equilibrium across the stack as the EA of HATCN (reported between 4.9 eV and 5.4 eV33,34) is higher than the perovskite Φ. In such case, the acceptor lowest unoccupied molecular orbital (LUMO) level is said to be pinned at the substrate EF. Monitoring the VB evolution upon increasing HATCN film thickness also reveals that the perovskite-related features progressively become attenuated and shift to lower BE. From 8 Å on, the HATCN HOMO can be observed with its peak maximum at ca. 4.90 eV BE. Noteworthy, a density of states (DOS) at ca. 1.8 eV BE (peak maximum) is detected for HATCN thickness ranging from 2 Å up to 32 Å. As this DOS is present neither for the pristine perovskite spectrum nor for that of the thick HATCN film, it is ascribed to an interface state, most likely corresponding to the HATCN LUMO that gets (partially)


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filled upon charge transfer.35

Figure 1. a) Secondary electron cutoff (SECO), b) valence band (VB) and core level spectra of c) Pb 4f and d) I 3d5/2 as a function of HATCN film thickness on pristine (low-DoSS) perovskite (MAPbI3-xClx) thin film. L* indicates the (partially) filled LUMO of HATCN molecules. The VBM of the pristine perovskite is determined to be at 1.50 eV, indicating a strong n-type surface. Dashed lines in the core level spectra are guides to the eye and the core level shifts are indicated as “∆”.

The observed work function change (∆Ф) has two contributions: i) a change in the perovskite surface band bending (∆ФBB), and ii) the formation of an interface dipole (∆ФID) due to charge transfer, such that ∆Ф = ∆ФBB + ∆ФID.36,37 ∆ФBB can be straightforwardly determined by monitoring the energy shifts of the perovskite VB and core levels upon molecular adsorption. Here, ∆ФBB is quantified from the Pb 4f and I 3d5/2 energy shifts as shown in Figure 1c and d. Both core levels exhibit an abrupt shift towards lower BE within 4 Å HATCN nominal thickness, indicating an upward band bending of 0.60 eV and implying that essentially flat band condition within the perovskite sample is established. For larger thicknesses, the core levels


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remain at essentially constant energy. Note that all the reported band bending values (and in the following graphs) were determined from the I 3d5/2 energy shifts in order to have the highest surface sensitivity (electron mean free path less than ~ 2 nm). To assess the general character of the ELA trend when forming interfaces between low-DoSS perovskite and acceptor molecules, the stronger electron acceptor F6TCNNQ with an EA of ca. 5.6 eV38 is further employed. The corresponding UPS and XPS results, displayed in Figure 2, are globally similar to those obtained with HATCN and are briefly described in the following. Upon incremental deposition of F6TCNNQ onto the pristine low-DoSS perovskite film, Φ increases monotonically and saturates at a nominal thickness of 16 Å with a final value of 5.36 eV (Figure 2a). Concomitantly, the Pb 4f and I 3d5/2 core levels as well as the perovskite valence features shift to lower BE. As shown in Figure 2d, the shift of the I 3d5/2 core level corresponding to ∆ФBB, indicates a 0.71 eV reduction of the surface band bending. The slightly larger ∆ФBB observed presumably relates to the higher EA of F6TCNNQ as compared to that of HATCN, which would therefore induce more electron transfer across the interface. Note that, with a transport bandgap of 1.70 eV32 and an initial VBM at 1.48 eV, flat band condition at the perovskite surface is readily reached and even slight upward band bending may persist. Finally, in analogy to the HATCN case, two prominent states are observed with their peak maximum at 2.8 eV and 1.5 eV BE upon F6TCNNQ adsorption and they are attributed to the molecular (relaxed) HOMO and partially filled LUMO.39,40 It is usually observed that charged molecules feature a reduced energy gap upon photoionization because of the Coulomb interaction due to


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the additional charge.39-41 In addition, the determined energy gap of 1.3 eV is consistent both with previous theoretical calculations of the density of states of partially charged F6TCNNQ molecules and with an experimental determination of the optical gap of F6TCNNQ anions.39,42 Note that the F6TCNNQ LUMO-derived state is even observed at high coverage as a result of the pronounced F6TCNNQ island growth on this substrate (see Supporting Information).

Figure 2. a) SECO, b) VB and core level spectra of c) Pb 4f and d) I 3d5/2 as a function of incrementally deposited F6TCNNQ on pristine perovskite (MAPbI3-xClx) thin film. H* and L* indicate the relaxed HOMO and (partially) filled LUMO of F6TCNNQ molecules, respectively. The VBM of the pristine perovskite is determined to be at 1.48 eV.

3.2. Energy Level Alignment at High-DoSS Perovskites/Acceptors Interfaces a) High-DoSS of Illuminated Perovskites Thin Films To establish the dependence of the ELA on the initial DoSS, further experiments were conducted using illuminated MAPbI3-xClx thin films as substrates. White light illumination of pristine perovskite thin films in UHV for 40 min allows creating


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high-DoSS stemming from Pb0 (coexisting with the expected Pb2+) as shown in Figure 3c and 4c. Such large Pb0 concentration in the surface region (also see VB region in Figure S1) induces strong EF pinning, which results in an even stronger n-type perovskite film character with ca. 0.2 eV energy shifts of the VBM and core levels to higher BE as compared to the low-DoSS substrates. Illumination treatment in UHV does induce partial precipitation of PbI2 within the perovskite. This is seen in the UV-Vis measurements by the emergence of a shoulder at ca. 500 nm, though the perovskite related features, and notably the optical absorption onset, are comparable to that of the pristine samples (see Figure S2).

Figure 3. a) SECO, b) VB and core level spectra of c) Pb 4f and d) I 3d5/2 as a function of incrementally deposited HATCN on illuminated perovskite (MAPbI3-xClx) thin film. L* indicates the (partially) filled LUMO of HATCN molecules. The VBM of the pristine perovskite, determined at 1.49 eV, increases after illumination to 1.65 eV, showing a strong EF pinning of the CBM. The strong low BE Pb 4f contribution is assigned to the emergence of Pb0.

Thickness-dependent UPS spectra of HATCN and F6TCNNQ on the illuminated


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high-DoSS perovskite thin films are shown in Figure 3 and 4 (a and b), respectively. Upon molecular adsorption, Φ increases from 4.24 eV to 5.51 eV (∆Ф = 1.27 eV) for HATCN, and from 4.17 eV to 5.60 eV (∆Ф = 1.43 eV) for F6TCNNQ, indicating electron transfer from the perovskite to the molecules. Most importantly, upon molecule adsorption the Pb0/Pb2+ ratio becomes strongly reduced, demonstrating that electron transfer occurs indeed mainly from the Pb0-related surface states. Also, this indicates that a control over the density of ionized donor-type surface states can be achieved by adsorption of electron acceptors, which might be useful for stabilizing interfaces in devices. As determined from the VB features and I 3d5/2 spectra, much smaller ∆ФBB of only 0.16 eV and 0.34 eV occurs upon deposition of HATCN and F6TCNNQ onto high-DoSS perovskite, respectively. The higher ∆ФBB as observed with F6TCNNQ is consistent with the stronger reduction of the Pb0/Pb2+ ratio upon adsorption of this molecule as compared to HATCN.

Figure 4. a) SECO, b) VB and core level spectra of c) Pb 4f and d) I 3d5/2 as a function of incrementally deposited F6TCNNQ on illuminated perovskite (MAPbI3-xClx) thin film. H* and L* indicate the relaxed HOMO and (partially) filled LUMO of F6TCNNQ molecules,


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respectively. The VBM of the pristine perovskite is determined to be at 1.61 eV, which after illumination increases to 1.71 eV, again showing strong EF pinning of the CBM.

b) High-DoSS of Single Crystal Perovskites Surfaces To further explore the influence of the DoSS, similar experiments were conducted using perovskite (MAPbI3) single crystals. Usually, single crystal perovskites are reported to have a lower defect density than polycrystalline films, which accounts for the large charge diffusion coefficient.43 However, we observe that single crystal perovskite surfaces display relatively high-DoSS (Pb0), resulting from the cleaving process under inert atmosphere before introduction into the UHV system (see Supporting Information). Also, a recent study by Yang et al.44 pointed out that density of surface defects is significantly higher for single crystals than for polycrystalline films. Molecular layer thickness-dependent UPS spectra of HATCN and F6TCNNQ on freshly cleaved single crystal surfaces are shown in Figure 5 and 6 (a and b), respectively. In both cases, Φ increases upon molecular deposition, from 4.35 eV to 5.43 eV (∆Ф = 1.08 eV) for HATCN, and from 4.24 eV to 4.80 eV (∆Ф = 0.56 eV) for F6TCNNQ. We remark that the small ∆Ф induced upon F6TCNNQ adsorption is related to the pronounced Volmer-Weber growth of these molecules37 on the wide terraces present on single crystal surface as observed by atomic force microscopy (see Supporting Information), which eventually results in a small fraction of the surface being actually covered, despite the large nominal layer thickness. This is in line with


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the weak attenuation of the perovskite core levels after F6TCNNQ deposition (see Figure 6c and d). As shown in Figure 5 and 6 (c and d) the Pb 4f and I 3d5/2 core levels of the single crystals exhibit only small energy shifts upon acceptor adsorption. Consistently, ∆ФBB of ca. 0.10 eV and 0.27 eV are found when the single crystals are covered with HATCN and F6TCNNQ, respectively.

Figure 5. a) SECO, b) VB and core level spectra of c) Pb 4f and d) I 3d5/2 as a function of incrementally deposited HATCN on freshly cleaved single crystal perovskite (MAPbI3) surface. L* indicates the (partially) filled LUMO of HATCN molecules. The VBM of the single crystal perovskite is at 1.51 eV.

Figure 6. a) SECO, b) VB and core level spectra of c) Pb 4f and d) I 3d5/2 as a function of


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incrementally deposited F6TCNNQ on freshly cleaved single crystal perovskite (MAPbI3) surface. H* and L* indicate the relaxed HOMO and (partially) filled LUMO of F6TCNNQ molecules respectively. The VBM of the single crystal perovskite is at 1.51 eV.

3.3. Discussion The ELA at similar perovskite/charge transport layer interfaces as reported by several research groups have shown pronounced differences, which have not been rationalized so far.18-22,45,46 This underlines the urgent need to establish a general framework, from which the different findings could be consistently explained and, more generally, from which the understanding of the electronic properties at perovskite/charge transport layer interfaces could be sensibly improved. The presented results demonstrate that the DoSS strongly impacts the ELA when forming interfaces with molecular acceptors, which can be used as electron transport layers in perovskite-based solar cells. This is summarized in Figure 7 where all determined ∆Ф and ∆ФBB values extracted from UPS and XPS measurements are presented.

Figure 7. a) and c) Evolution of work function change (∆Ф) and b) and d) band-bending (∆ФBB) as a function of HATCN (top) and F6TCNNQ (bottom) nominal thicknesses on pristine, single crystal and illuminated perovskites surfaces obtained from the above shown UPS and XPS results.


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Although the ∆Ф values upon adsorption of HATCN and F6TCNNQ on both low- and high-DoSS perovskites are overall similar (apart from the case of F6TCNNQ on the single crystal perovskite where the growth mode does not support full surface coverage), ∆ФBB as observed with low-DoSS perovskites is much higher than those with high-DoSS perovskites. As estimated from ∆ФBB, when using low-DoSS surfaces the initially weakly EF-pinned perovskite bands29 become essentially flattened in the surface region. Considering the initial downward surface band bending in the pristine perovskite samples, being induced by the donor-type Pb0 surface states, we propose that the band flattening is a consequence of the reduction of the ionized DoSS as electrons are transferred from the surface states to the molecular acceptors. This proposition is further supported by the observed decrease in Pb0 concentration when the acceptors are deposited onto the high-DoSS perovskite samples. In this case, due to the high-DoSS the CBM is very strongly pinned at EF and only small ∆ФBB values are observed. This interpretation is further consistent with the reported surface photovoltage effect that initially occurs for pristine perovskite samples but becomes fully quenched upon prolonged white light illumination as a result of the created additional DoSS. For the clear-cut case of HATCN, the interfacial energy level diagrams for different perovskite surface DoSS are depicted in Figure 8. The Fermi-level position is assumed to be at mid-gap in the bulk, which is consistently based on our previous study29 on the surface photovoltage effect of mixed halide perovskite thin films as


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mentioned above. Besides, XPS measurements in dark and under white light illumination were additionally performed on pristine perovskite thin film covered with 70 Å HATCN performed. As shown in Supporting Information (Figure S5), no energy shift of perovskite core levels could be observed during this experiment, which suggests that flat band condition is readily reached in the perovskite film after molecular deposition. However, as the electronic properties of bare perovskites are still debated, alternative interfacial energy level diagrams with initial flat band condition in the bare perovskite sample are discussed in the Supporting Information (Figure S6). These diagrams emphasize that different band bending scenarios within the perovskites are determined by the DoSS, which results in large energy level offset variations. For instance, it can be seen that the energy offset between the VBM and HOMO onset amounts to ~3.22 eV and ~2.68 eV when low- and high-DoSS perovskites are employed, respectively. This shows that the energetics at these interfaces are substantially modified by the perovskite DoSS, which, in turn, tremendously impacts the charge extraction process in perovskite devices, as can be seen for the case of electrons in Figure 8.


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Figure 8. Schematic energy level diagrams for the adsorption of HATCN acceptor on a) low and b) high-DoSS perovskite surfaces. The reduction in surface band-bending (∆ФBB) by 0.60 eV for low-DoSS and by 0.16 eV for high-DoSS perovskite surfaces upon acceptor deposition is indicated. Red shades indicate the filled DoSS.

4. CONCLUSIONS We investigated the impact of Pb0-derived surface state density on the interfacial ELA at perovskites/molecular electron acceptor heterojunctions. It is found that the initial downward surface band bending, induced by the Pb0 donor-like surface states, for pristine perovskite surfaces can be largely reduced upon adsorption of acceptor molecules. Depending on the initial DoSS, a variety of ELA situations can thus be achieved. Upon molecular acceptor adsorption, a flat band condition within the (initially n-type) perovskite can be readily reached for low-DoSS samples, as the core levels shift by ca. 0.7 eV towards mid-gap. In contrast, in the case of high-DoSS perovskites, strong EF pinning at the CBM results in a still strong interfacial n-type


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character even after acceptor deposition and concomitant electron transfer. Consequently, the DoSS directly relates to the ELA at interfaces with charge transport layers. Therefore, taking into account the DoSS, which can indeed vary considerably for different sample preparation conditions, should help to explain why dissimilar data and interpretation of the energetics are reported for nominally identical interfaces. Hence, it will be of high importance to further investigate how different types of defects present in perovskites can influence the interface energetics in order to better understand the working mechanism in relevant devices, and to aid device design for boosting their performance. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. UPS spectra of at Fermi-level region on different perovskites surfaces with varying DoSS, absorption spectra of pristine and illuminated perovskite films, AFM images of the pristine, illuminated and single crystal perovskites before and after F6TCNNQ and HATCN deposition, high resolution XPS spectra, XPS measurements on effect of surface photovoltage for HATCN covered pristine perovskite thin film, alternative energy level diagrams, pole-figure measurements on a cleaved single crystal perovskite (PDF) AUTHOR INFORMATION Corresponding Author


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*P. A. email: [email protected] *N. K. email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the Helmholtz Energy Alliance "Hybrid Photovoltaics", the Joint Graduate School HyPerCells of the University of Potsdam and the Helmholtz Zentrum Berlin, and the DFG (Sfb951). P. A. acknowledges the DFG (AM 419/1-1) for financial support. REFERENCES (1) Snaith, H. J. Perovskites: The Emergence of a New Era for Low-Cost, High-Efficiency Solar Cells. J. Phys. Chem. Lett. 2013, 4, 3623-3630. (2) Park, N.-G. Organometal Perovskite Light Absorbers Toward a 20% Efficiency Low-Cost Solid-State Mesoscopic Solar Cell. J. Phys. Chem. Lett. 2013, 4, 2423-2429. (3) Grätzel, M. The light and shade of perovskite solar cells. Nat. Mater. 2014, 13, 838-842. (4) Stolterfoht, M.; Wolff, C. M.; Amir, Y.; Paulke, A.; Perdigón-Toro, L.; Caprioglio, P.; Neher, D. Approaching the fill factor Shockley–Queisser limit in stable, dopant-free triple cation perovskite solar cells. Energy Environ. Sci. 2017, 10,


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Surface State Density Determines the Energy Level Alignment at

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